The present invention relates to a channel selection apparatus for use as a cable television (referred to in the following as CAATV) converter, which utilizes a double superheterodyne (abbreviated in the following to double superhet) tuner circuit.
In recent years the up-down converter type of double superhet circuit has come into widespread use as the tuner circuit in CAATV converters. Such a CAATV converter functions to select one of a number of channel signals contained in an input signal, to be produced as a fixedly predetermined television channel signal which is supplied, for example, to a television receiver. The input signal is applied to an up-converter section which produces a first IF signal by heterodyning the input signal up to a higher frequency, and this first IF signal is supplied to a down-converter section to be converted to a second IF signal which is lower in frequency then the first IF signal. The up-converter section includes a first local oscillator whose frequency of oscillation is varied to execute selection of a required channel, while the down-converter section includes a second local oscillator whose frequency of oscillation is held fixed, irrespective of the selected channel.
With such a system it is generally necessary to employ a phase lock loop (abbreviated in the following to PLL) or automatic frequency control (abbreviated in the following to AFC) circuit to control the local oscillator frequencies, in order to ensure stability of the selected channel frequency.
An example of a prior art channel selection apparatus of the type described above is illustrated in FIG. 1, in which the portion 15 enclosed by a broken-line outline is a tuner section, formed of an up-down converter double superhet circuit. The tuner section includes an input filter 1, a first mixer 2 and a first local oscillator 7. An input signal containing a number of television channels as described above is transferred through the input filter 1 to be mixed in the first mixer 2 with a local oscillator signal produced from the first local oscillator 7, to be thereby converted to a first IF signal. This first IF signal is transferred through a band-pass filter 3 and then amplified in a first IF amplifier 4, which produces an output signal that is supplied to a second mixer 5. A second local oscillator 9 produces a local oscillator signal which is mixed with the first IF signal in the second mixer 5, to thereby convert the first IF signal to a second IF signal. The second IF signal is then amplified in a second IF amplifier 6, whose output (generally referred to as the video IF signal) is applied to an output terminal D. In this example, both the first local oscillator 7 and the second local oscillator 9 are voltage-control oscillators. Numeral 8 denotes a prescaler, which executes frequency division of the output signal from the first local oscillator 7. Numeral 10 denotes a VIF (video IF) amplifier, and 11 an automatic frequency control (AFC) circuit. Numeral 12 denotes a LPF (low pass filter), 13 denotes a PLL (phase lock loop) circuit, and 14 denotes a microcomputer.
The operation of this prior art channel selection apparatus is as follows. The video IF signal produced from the tuner section 15 is amplified in the video IF amplifier 10, and the resultant output signal from the video IF amplifier 10 is applied to the AFC circuit 11. The AFC circuit 11 thereby produces a frequency control voltage in accordance with the frequency of the video IF signal, which is supplied through an input terminal C of the tuner section 15 to control the frequency of oscillation of the second local oscillator 9. In this way, closed loop control is executed to stabilize the frequency of oscillation of the second local oscillator 9.
As stated above, the second local oscillator 9 is set at a fixed frequency of oscillation. However there is a possibility that this frequency will drift, ue to changes in temperature etc., by up to 500 KHz, for example, whereas it is necessary that the frequency of oscillation of the second local oscillator 9 be stable to within .+-.50 KHz. If the frequency of oscillation should depart from that range, changes such as generation of spurious beat components will occur in the video IF signal produced from the apparatus, which will affect the displayed television picture. For this reason it is preferable to employ an AFC loop to stabilize the frequency of oscillation of the second local oscillator 9 as described above.
The first local oscillator 7 on the other hand must have a capability for varying the frequency of oscillation thereof, in order to select various channels contained in the input signal. Such alteration of the frequency of oscillation of the first local oscillator 7, i.e. channel tuning operation, is executed by varying a control voltage which is applied to the input terminal A of the tuner section 15. Since the frequency of oscillation of the first local oscillator 7 is high, and the changes which are made in that frequency in order to execute channel selection are relatively large, a PLL circuit 13 is used to stabilize and to control changes in that frequency of oscillation. The output signal from the first local oscillator 7 is frequency divided in the pre-scaler 8, and the resultant frequency-divided signal is applied from terminal B of the tuner section 15 to the PLL circuit 13. Tuning data, to designate selection of a desired television channel, are produced from the microcomputer 14 in response to actuations of an input device such as a keyboard or a remote control unit (not shown in the drawings), and are transferred to the PLL circuit 13. The PLL circuit 13 contains a frequency divider which executes frequency division of the output signal from the pre-scaler 8 by a division ratio 1/n, with this frequency division ratio being determined by the tuning data from the microcomputer 14. The resultant frequency-divided signal is compared in the PLL circuit 13 with a reference frequency, to detect any deviation of the frequency-divided signal, and a detection signal representing such a deviation is transferred through the low-pass filter 12 to the input terminal A of the tuner section 15 and hence supplied to the first local oscillator 7 as a frequency control voltage. The frequency of oscillation of the first local oscillator 7, which is a voltage-control oscillator, is thereby controlled such as to compensate for the aforementioned frequency deviation, i.e. closed-loop control of the first local oscillator 7 is executed.
However in recent times it has become necessary to utilize such a channel selection apparatus for a variety of applications, including television games for example, as well as multi-channel CAATV operation, in which the transmission frequency of a television channel selected by the apparatus may have a significant offset from the standard frequency for that channel. In such a case, with for example an offset of 1 MHz, it is desirable that means be provided for automatically adjusting the frequency of the first local oscillator 7 by an amount which will compensate for the offset. This could be done in principle by detecting such a transmission frequency deviation, sending the resultant detection information to the microcomputer 14, and arranging that the microcomputer 14 commands the PLL circuit 13 to execute the requisite amount of change in the frequency of oscillation of the first local oscillator 7, e.g. with data sent from the microcomputer 14 causing the PLL circuit 13 to execute one or more step changes in the frequency of oscillation of the first local oscillator 7 such as to produce the offset compensation described above, so that follow-up control of the first local oscillator is performed.
This type of AFC operation will be further described referring to FIG. 2, which shows the AFC characteristic of the AFC circuit 11 in FIG. 1, i.e. the relationship between the frequency of the second IF signal applied from the video IF amplifier 10 and the resultant control voltage V.sub.c which is produced from the AFC circuit 11 and supplied to the second local oscillator 9. Assuming that both the first local oscillator 7 and the second local oscillator 9 are operating at the respective standard conditions for each of these for selecting a television channel of a specific transmission frequency, but that there is an offset of that transmission frequency from the standard frequency thereof, then the output voltage V.sub.c produced from the AFC circuit 11 might for example correspond to point 1 indicated on the characteristic of FIG. 2. The voltage range from V.sub.1 to V.sub.2 in FIG. 2 will be assumed to constitute a "window", within which the AFC circuit 11 can perform AFC operation to hold the second IF signal close to the standard value thereof, i.e. this represents the detection range for AFC control by the AFC circuit 11. Thus, if detection means were provided for indicating to the microcomputer 14 that the second IF signal frequency is outside this window range, it could be arranged that the microcomputer 14 will supply control data to the PLL circuit 13 whereby one or more step changes in frequency of the frequency of oscillation of the first local oscillator 7 (and hence of the frequency of the second IF signal) are executed, until the window range W is entered. When it is detected that the window range W has been entered (by the aforementioned detection means), then the microcomputer 14 could be notified accordingly, to thereby halt any further frequency stepping operations. Thus for the case of the control voltage V.sub.c being initially at position 1 in FIG. 2, a first step in frequency of the first local oscillator 7 would be executed to position 2, then (since the control voltage V.sub.c is still outside the window W) a second frequency step would be executed to bring the voltage V.sub.c to position 3, which is within the window W. Frequency stepping would then be terminated.
Thus if it were possible to utilize such a control voltage (i.e. from the AFC circuit which controls the second local oscillator frequency) for detecting when the transmission frequency has deviated from the standard value thereof, it would be possible for the converter to follow, i.e. to compensate for such a deviation. However in the prior art, this frequency control voltage is continuously applied to stabilize the frequency of oscillation of the second local oscillator, so that such compensation for a deviation of the transmission frequency is impossible to implement.